1. Field of the Invention
This invention relates to an optical analog-digital (hereinafter abbreviated as A-D) converter, and in particular to a highly accurate optical A-D converter capable of accomplishing optical parallel processing utilizing two kinds of output lights output from a nonlinear optical element.
2. Related Background Art
In recent years, in the field of image processing and the field of image communications, it has been practised to use a so-called A-D converter for converting analog image information regarding an image into optical or electrical digital information and digitally process the digital information obtained from the A-D converter by a computer with high accuracy and at a high speed.
Particularly, the field of image processing has been remarkably developed and is looking for a demand for diverse A-D converters of high speed and high accuracy.
Generally, in the image processing, an attempt to achieve higher accuracy leads to an increase in the number of picture elements of an image. On the other hand, recently, the amount of information in an image has increased. Accordingly, it is required that the processing time per picture element be made higher than before.
For example, mentioning a specific example of the numerical value, the processing speed of the A-D converter for 10 bits is now about 0.1 .mu.sec. in the case of a considerably high speed converter.
If the number of picture elements to be processed is, e.g. 100.times.100, a processing time of about 1 msec. is required for the entire image. Also, if the number of picture elements to be processed is 1000.times.1000, the processing time required will be considerably long, say, about 100 msec.
Therefore, recently, attempts to achieve a higher speed by utilizing optical processing instead of electrical processing have been proposed, for example, in Japanese Laid-Open Patent Applications Nos. 54-11756 and 56-153921. In these publications, electrical analog information is replaced with an optical signal and at the same time, quantization is effected by optical processing using a density filter or the acousto-optical effect and binarization is effected as an electrical signal in conformity with the quantization levels.
In these publications, however, the optical signal is used only at the stage of quantization and therefore, photoelectric conversion elements, code generating circuits, etc. corresponding in number to the number of quantization levels are required at the stage of binarization. Thus, if the number of quantization levels is increased in conformity with the number of bits to be processed, the construction becomes correspondingly complex, and also, the optical processing is used only in a portion, and this has led to difficulty in achieving an overall high speed.
Also, a method of effecting A-D conversion without the use of electrical chips is proposed, for example, in U.S. Pat. No. 4,565,924. In this patent, however, an optical analog quantity is simply distinguished in two stages and the method of this patent is limited in its use.
As noted above, in the image processing according to the prior art, it has been very difficult to achieve a high speed by optically effecting A-D conversion.
On the other hand, so-called nonlinear optical elements emitting an output light having a nonlinear characteristic relative to the intensity of an input light are discussed in "All-optical computation--a parallel integrator based upon a single gate full adder", B. S. Wherrett, Optics Communications, Vol. 56, No. 2, pp. 87-92, Nov. 15, 1985, "Optical bistability, photonic logic, and optical computation", S. D. Smith, Applied Optics, Vol. 25, No. 10, pp. 1550-1564, May 15, 1986, and "Application of bistable optical logic gate arrays to all-optical digital parallel processing", A. C. Walker, Applied Optics, Vol. 25, No. 10, pp 1578-1585, May 15, 1986.